Adsorption of some cationic dyes onto two models of graphene oxide

Context: The search for highly efficient adsorbent materials remains a significant requirement in the field of adsorption for wastewater treatment. Computational study can highly contribute to the identification of efficient material. In this work, we propose a computational approach to study the adsorption of four cationic basic dyes, basic blue 26 (BB26), basic green 1 (BG1), basic yellow 2 (BY2), and basic red 1 (BR1), onto two models of graphene oxide as adsorbents. The main objectives of this study are the assessment of the adsorption capacity of the graphene oxide towards basic dyes and the evaluation of the environmental and temperature effects on the adsorption capacity. Quantum theory of atoms in molecules (QTAIM) analysis has been used to understand the interactions between the dyes and graphene oxides. In addition, adsorption free energies of the dyes onto graphene oxides are calculated in gas and solvent phases for temperatures varying from 200 to 400 K. As a result, the adsorption free energy varies linearly depending on the temperature, highlighting the importance of temperature effects in the adsorption processes. Furthermore, the results indicate that the environment (through the solvation) considerably affects the calculated adsorption free energies. Overall, the results show that the two models of graphene oxide used in this work are efficient for removing dyes from wastewater. Methods: We have optimized the complexes formed by the interaction of dyes with graphene oxides at the PW6B95-D3/def2-SVP level of theory. The SMD solvation model realizes the implicit solvation, and water is used as the solvent. Calculations are performed using the Gaussian 16 suite of program. QTAIM analysis is performed using the AIMAll program. Gibbs free energies as function of temperature are calculated using the TEMPO program. Supplementary Information The online version contains supplementary material available at 10.1007/s00894-023-05761-8.


Synthetic dyes
They are made from man-made chemicals.They are used in many consumer products such as clothing, cosmetics and textiles.From these raw materials, the intermediates are made by a series of chemical processes which, in general, correspond to the replacement of one or more hydrogen atoms of the starting material by particular elements or radicals 2 .These are compounds having molecules such as benzene as their basic structure.
Structure plays an important role in determining the coloring properties of organic compounds.In general, a typical dye molecule consists of three parts: a chromophore, an auxochrome group, and a solubilizing group.The chromophore is somehow the portion responsible for the color of the compound.The auxochrome is the part influencing the intensity of the coloring and it effectively fixes the dye on the support and finally the solubilizing group improves the solubility of the dye and thus, it can potentially be applied in aqueous medium 3 .Some types of dyes are studied in our work: see table1

Environmental effects of dyes
Dyes can be toxic to living organisms if not properly removed, which can lead to ecosystem disturbances and death of living organisms.They can also cause water and soil pollution when released into industrial wastewater.They can also have a negative impact on human health, with some dyes being classified as carcinogenic 4,5 .The production of certain dyes can also have a negative impact on the environment due to the use of toxic chemicals such as organic solvents and heavy metals.

The adsorption mechanism of a dye
Adsorption is a process, widely answered for dye removal also has wide applicability in wastewater treatment 6 .Separation by adsorption is based on selective adsorption (thermodynamic and/or kinetic) of pollutants (called adsorbate) by an adsorbent, thanks to specific interactions between the surface of the material and the adsorbed products 7 it is a simple mass transfer from liquid phase towards the surface of the solid, this process takes place in several stages.
External diffusion: corresponds to the transfer of the solute (a dye) from within the solution to the external surface of the grains.The transfer of external matter depends on the hydrodynamic conditions of the flow of a fluid in an adsorbent bed.
Internal diffusion: the fluid particles penetrate inside the pores.It depends on the concentration gradient of the solute.Diffusion of the surface in contact with the active sites, it corresponds to the fixing of the molecules on the pore surface.3 The structure of graphene oxide Compared to graphene, graphene oxide or GO is strongly oxygenated by hydroxyl and epoxide groups on sp 3 hybridized carbons in the basal plane, as well as carbonyl and carboxyl groups located at the edges of the sheets on the hybridized carbons sp 2 (figure 1).Graphene oxide has many interesting physicochemical properties that make it an innovative material 8 : Structure: Graphene oxide is composed of oxidized carbon layers, forming a sheet structure with oxygen atoms bonded to carbon atoms.It retains the sheet structure of graphene, but with a surface modified by the groups functional.
Conductivity: Although graphene oxide is an oxidized form of graphene and therefore less conductive, it still retains some electrical conductivity, which distinguishes it from other carbon oxides.
Thermal stability: Graphene oxide exhibits high thermal stability, making it resistant to high temperatures without degrading.This makes it a promising material for high temperature applications.
Reactive surface: The functional groups present on the surface of graphene oxide make it very chemically reactive.It can react with different molecules and be functionalized for specific applications.
Storage capacity: Graphene oxide has a high adsorption capacity, which means that it can absorb and hold different molecules on its surface.This makes it attractive for applications such as gas detection, water purification and energy storage.The functional PW6B95D3 9 refers to a specific combination of functionals in the density functional theory (DFT) method used in quantum chemistry.It combines the terms PW6 (perdew-wang 2006) and B95 (Becke 95) with the parameter D3 to include dispersion corrections according to the Grimme method.The PW6 functional is known for its good behavior towards electronic properties in various chemical systems, while the B95 functional offers a better description of ionization and electron affinity energies.Finally, the dispersion correction D3 makes it possible to account for dispersion interactions, which are essential for an accurate description of molecular systems.Def2SVP 10 is a basis set (or slater basis) which is used to represent atomic orbitals in electrical calculations.This base set (doublezeta) includes two Gaussian functionals for internal electrons (core), four functional functions for valence electrons and two Gaussian functions for polarizable electrons.The interest of the PW6B95D3/def2SVP functional lies in the fact that it provides an accurate description of electronic interactions, taking into account both exchange and correlation interactions.It has been parameterized to be used with the SVP polarization deficient basis (def2SVP), which allows faster calculations while maintaining good precision.

Analysis of frontier orbitals
The energies of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are very popular descriptors of quantum chemistry.energy of the HOMO orbital describes the ability of the molecule to donate electrons.Conversely, the energy of the LUMO orbital describes the ability of the molecule to accept electrons 11 .Frontier molecular orbitals play an important role in electrical, optical and chemical properties 12 .Figure 2 shows the HOMO and LUMO orbitals of the basic dyes tested.

4 functional hybrid used S2 a model 1 b model 2 Figure S1
Figure S1 Structure of graphene oxide

a
Figure S2 HOMO and LUMO frontier molecular orbitals of the four dyes investigated in this work at the PW6B95-D3/def2-SVP computational level of theory.

Table S1
Studied dyes and their chemical structures.